The species composition and diversity of crab assemblage in Haizhou Bay was assessed using bottom-trawl survey data collected in March, May, July, September, and December, 2011. Crab species composition and the spatio-temporal variations in species diversity of communities were examined using ecological diversity indices. In total, 34 crab species belonging to 18 families and 27 genera were caught during the surveys. There were 16 warm water species, 15 warm temperate species and 3 cold temperate species. Margalef’s species richness index (D) was highest in March and lowest in December while the Shannon-Wiener diversity index (H′) and Pielou evenness index (J′) were highest in July and lowest in December. Diversity, richness, and evenness also varied spatially over time. In March, May and July the highest values occurred in the northern area of the survey site and lowest in the southern part. In September, values were lowest in the central survey site and high in the northern and southern areas. In December, values were low in northern survey site and high in southern areas. The number of individuals per haul per hour varied spatially and temporally by month with higher numbers in March, May and December. Correlation analysis showed that crab community species diversity was significantly affected by water temperature, salinity, depth and the season in Haizhou Bay. The spatio-temporal variations in species composition and diversity of crab assemblage in Haizhou Bay are primarily related to the seasonal changes in environmental factors such as bottom water temperature and the spatial distribution of main dominant species.